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Astron. Astrophys. 341, 768-783 (1999)

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3. General principles of the equivalent width method

The presence of a magnetic field enhances the equivalent widths (EWs) of certain spectral lines. Deriving the magnetic field strength from the EWs was first pioneered by Hensberge and de Lore (1974), who used lines from the flat part of the curve of growth for which the amplification should be proportional to the magnetic sensitivity [FORMULA], where [FORMULA] is the effective Landé factor of the line transition, and [FORMULA] its wavelength. This method was later refined by Basri et al. (1992) who showed that the amount of intensification not only depends on [FORMULA] but also on the intrinsic Zeeman pattern of the line. The advantage of this method is that, at least in the optical, it is more accurate than measuring the width of the lines, especially for rapidly rotating stars. Also, blends are less of a problem. However, a full radiative transfer code for the stellar atmosphere, and highly accurate gf-values are needed.

The basic idea is to fit the EW of all useful lines by a superposition of field free regions (with weight [FORMULA]) and with regions of field strength B and inclination to the line of sight [FORMULA] (with weight [FORMULA]), and to find the B and [FORMULA] for which the best fit occurs. The values of the weights [FORMULA] and [FORMULA], and hence the filling factor f, then follow from this best fit (see Eqs. 7 - 10).

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© European Southern Observatory (ESO) 1999

Online publication: December 16, 1998